This invention relates to phenylene ether resins and more particularly to an improved method for preparing polyphenylene ether resins having carboxyl functionality.
Polyphenylene ether resins (PPE) have long been known for use as high temperature thermoplastics. For example, PPO, or poly(2,6-dimethyl-1,4-phenylene ether), was disclosed and described in U.S. Pat. Nos. 3,134,753 and 3,306,874. More recently, in U.S. Pat. No. 4,011,200, phenylene ether copolymer resins having improved thermal stability were described. These resins have achieved a high degree of acceptance, particularly for use in further blends with other polymers such as styrenic resins, described for example in U.S. Pat. Nos. 3,356,761; 3,383,435 and 4,038,543. Polyphenylene ethers are substantially non-polar in character, and are therefore difficult to blend with a great many of the more polar resins such as polyamides and polyesters. Moreover, the adhesion of PPE resins to a variety of conventional reinforcing materials such as glass fiber is often poor, and modification of the surfaces of such materials may be required in order to achieve adequate reinforcement of PPE resins.
Methods for modifying the non-polar character of PPE resins are known. The use of functional comonomers, as well as the use of metalation, sulfonation, chloromethylation and similar processes have been described for introducing reactive functionality into such resins. Treating preformed polyphenylene ethers with a combination of a styrenic monomer and maleic anhydride in the presence of a free-radical initiator is shown in U.S. Pat. No. 4,097,556 to provide polyphenylene ether-styrene-maleic anhydride graft copolymers which are said to be useful in blends with polyamides. Processes for directly attaching maleic anhydride to phenylene ether resins in the presence of a peroxide are shown in published Japanese applications Nos. 59/66452 and 59/59724. Blends of these maleated phenylene ether resins with polyamides are also disclosed therein, and the carboxylated resins are said to have improved affinity for glass.
Although the methods available for carboxylating phenylene ether resins and polyamides appear to be successful, further improvements are needed. Chemical modification of phenylene ether resins, either by use of functional comonomers or in a post-reaction, requires additional and costly process steps. The methods presently known for directly modifying phenylene ether resins generally require extended mixing times at melt processing temperatures and/or the use of free-radical compounds, conditions which tend to promote crosslinking and/or deterioration of the resin. Extended mixing at high temperatures also increases energy consumption and adds to production costs. An improved method for directly carboxylating phenylene ether resins which substantially reduces processing times and minimizes resin cross-linking and degradation is needed.